In recent years, there have been developed polymer electrolyte fuel cells which can be operated in a temperature range of from ordinary temperature to around 80° C. by using as an electrolyte membrane a polymer electrolyte membrane which is a proton conductor. Polymer electrolyte fuel cells have a wide variety of applications such as power generation systems for domestic power supply, distributed power supply and the like, cogeneration systems combining the above power generation systems with waste heat recovery technology, power supply for driving mobile bodies such as automobiles, and power supply for mobile terminals of electronic equipment and the like.
In order to commercialize polymer electrolyte fuel cells, higher output is desired. Increase of the operation temperature of fuel cells, specifically to 100° C. to 200° C., is considered to be effective for this purpose, because increase of operation temperature can increase the efficiency of power generation of the cells and improve the output of the fuel cells.
Polymers typically used as a proton conductor for polymer electrolyte fuel cells at present are fluorocarbon polymers having a strong acid functional group in a side chain, that is, perfluorosulfonic acid polymers. Among them, Nafion® manufactured by EI du Pont de Nemours and Company is the most typical one. Perfluorosulfonic acid polymers have the advantage that, by humidification, they have a very high proton conductivity of around 10−1 S/cm in a temperature range of from ordinary temperature to 100° C.
However, perfluorosulfonic acid polymers have the disadvantage that their proton conductivity is greatly reduced at a temperature of 100° C. or more. Therefore, it is impossible to use them in a high temperature environment. Perfluorosulfonic acid polymers retain water by humidification, and this water forms an ion-conducting path. Proton conductivity is greatly reduced because the water forming the ion-conducting path is evaporated at a temperature of 100° C. or more.
In order to solve the problem as described above, use of a basic polymer doped with a strong acid as a proton conductor is disclosed (for example, refer to Patent Document 1). This proton conductor is a basic polymer such as polybenzimidazole doped with a strong acid in liquid form such as sulfuric acid or phosphoric acid.
Patent Document 1 describes the “doping” as follows. When basic polymers are doped with strong acids, the strong acids are dissociated into protons and acid anions. The basic polymers receive protons dissociated from the acids and are protonated. The protonated basic polymers form acid-base bonding with the acid anions. The thus formed proton conductors exhibit a high proton conductivity of 10−2 S/cm or more in a temperature range of from 100° C. to 200° C. even under low humidity.
However, since the proton conductor described in Patent Document 1 is obtained by “doping” a basic polymer with an acidic substance, the bonding strength between the basic polymer and the acidic substance is not necessarily sufficient. Therefore, there is a problem that the proton conductivity is prone to be reduced due to the elimination of the acidic substance from the proton conductor. It is likely that the performance of the fuel cells using a proton conductor as described above is prone to be reduced (for example, refer to Non-Patent Document 1).
Moreover, there is proposed a proton conductor obtained by impregnating an acidic polymer (or a basic polymer) with a basic polymer (or an acidic polymer) so as to form acid-base bonding (for example, refer to Patent Document 2). It is described that this proton conductor exhibits a proton conductivity of from 10−3 to 10−2 S/cm at 150° C. in a low-humidity condition (in a nitrogen stream). It is likely that the reduction of proton conductivity due to the elimination of acid or base is alleviated since both the acid and base are polymers in the proton conductor described in Patent Document 2.
However, the proton conductor described in Patent Document 2 has a problem that its proton conductivity does not attain a practical level in a temperature range of from 100 to 200° C. This may be due to the fact that, since both the acidic and basic substances are polymers, it is impossible to sufficiently increase the content of the acidic substance or the basic substance in the proton conductor. Generally, the mechanical strength and chemical stability of the polymer containing an acidic or a basic functional group are reduced as the amount of the functional group is increased.
An electrolyte membrane may be prepared from a proton conductor having a low mechanical strength, but problems such as breakage may occur since the strength of the membrane is weak. Further, when chemical stability of a proton conductor is reduced, problems such as elution of polymers at a high temperature or dissolution thereof in water may become conspicuous. Furthermore, there is also a problem that the type of polymers that can maintain heat resistance is limited in a temperature range of from 100 to 200° C.
Moreover, there is also disclosed a proton conductor containing an acidic polymer, a basic polymer and an elastic polymer (for example, refer to Patent Document 3). By incorporating an elastic polymer, it is possible to obtain a proton conductor having an increased mechanical strength compared to a proton conductor consisting only of an acidic polymer and a basic polymer.
However, although the proton conductor described in Patent Document 3 improves mechanical strength, it does not alleviate the reduction of chemical stability. From the viewpoint of maintaining chemical stability, the amount of an acidic substance or a basic substance contained in the proton conductor cannot sufficiently be increased. The result is that the proton conductivity in a temperature range of from 100 to 200° C. also may not attain a sufficiently high level.    [Patent Document 1]: National Publication of International Patent Application No. 1999-503262    [Patent Document 2]: Japanese Patent Laid-Open No. 2001-236973    [Patent Document 3]: National Publication of International Patent Application No. 2003-535940    [Non-Patent Document 1]: Electrochemistry, Vol. 70, No. 12, p. 943-945 (2002)